Etretinate Blood Levels in Monitoring of Compliance and Contamination in a Chemoprevention Trial

We measured serum etretinate to monitor compliance in an ongoing chemoprevention trial in which heavy smokers are randomized to either etretinate or placebo orally for 6 months. Blood is collected for determination of etretinate levels before treatment and then at 2, 4, 8, 16, and 24 weeks after randomization. The monitoring strategy was assessed by interim evaluation. There were 276 posttreatment samples available from 75 randomized subjects of whom 36 received etretinate and 39 placebo. The mean coefficient of variation for the internal standard retinyl acetate in serum was 4.16% for the high-pressure liquid chromatography method used. Among positive samples, the mean etretinate concentration was 25.7 ng/mL (SD, 23.4). Of the 131 samples obtained from subjects randomized to etretinate, 120 or 91.6% had detectable levels compared with 4 of 145 or 2.8% placebo samples. Among the 36 subjects given etretinate, at least one positive test occurred. In 27 of these 36 participants, etretinate was detected in every sample obtained. In the other nine, the absence of drug could be explained by pill counts or a history of discontinuation of treatment for six. Among the 39 subjects given placebo, the four positive samples were from four individuals, all of whom were negative on three other occasions. These data confirm the usefulness of the monitoring system we used and indicate that compliance and/or contamination will not be major problems in this trial.